a) Field of the Invention
This invention relates to a Ni--P electroless-plated magnetic hard disc substrate made of an aluminum alloy which is a high capacity-type, such as those using a MR head or the like, and a process for manufacturing the magnetic disc.
b) Description of the Prior Art
A magnetic hard disc substrate can be generally obtained by punching a roll coil made of an aluminum alloy to prepare a blank, grinding the blank to remove flaws, waviness, and the like to obtain a substrate, then electroless plating the substrate with Ni--P, finish-polishing the plated substrate, then forming a magnetic film by magnetic sputtering, and coating the magnetic film with an overcoat to obtain a sputtering media. In order to allow the surface roughness and the flatness to fall in prescribed ranges respectively in these steps, the grinding of the magnetic hard disc substrate (blank) prior to the Ni--P electroless plating is performed by the following method: the substrate is put onto a polishing board to which a nonwoven fabric polishing cloth made of an organic polymer is applied and (a) the polishing operation is performed under a fixed pressure while supplying, to the polishing face, a polishing solution produced by dispersing metal oxide particles, such as alumina, titania, and zirconia with an average grain size of 0.3 to 5 .mu.m in an organic acid-type etchant wherein the polishing operation consists of a first stage polishing using abrasive grains of a larger size and successively second stage polishing using abrasive grains of a smaller size; or (b) the polishing operation is performed using a polishing solution produced by dispersing colloid particles of silica, zirconia, titania, or the like, with an average grain size of 0.01 to 0.3 .mu.m in an acid-type or alkali-type etchant.
In general, the magnetic hard disc substrate made of an aluminum alloy which is electroless plated with Ni--P and is used for a hard disc of memory devices, such as a computer, is improving in recording density from year to year. There is a great demand for more improvement in the recording density. There is also a tendency to a to higher density and larger capacity. It is therefore important to finish the magnetic hard disc substrate used in these fields so that the substrate has the prescribed surface roughness and flatness. Particularly, the realization of high density recording is due to a remarkable progress in low floatation technologies based on the improvement of the head, which requires the reduction of the interval between the head and the medium. With the reduction in the interval, the magnetic disc needs to have a smooth surface and mostly reduced surface defects. Specifically, the following polishing qualities are required: the surface roughness Ra.ltoreq.5 angstroms, the surface roughness Rmax.ltoreq.80 angstroms, and the surface is free from scratches with a depth of 50 angstroms or more and from pits with a depth of 50 angstroms or more. In the conventional method in which the polishing is performed using a polishing solution containing abrasive grains composed of a metal oxide such as alumina, titania, or zirconia, particles with a large grain size cannot be prevented from getting slightly mixed therein during a classification stage because these oxides are produced by grinding a massive raw material and classifying the ground material. The contaminant particles with a large grain size cause the production of scratches with a depth of 50 angstroms.
In the method (a) among the aforementioned conventional methods, though the waviness of the substrate decreases, the obtained value of Ra is about 10 angstroms and polishing flaws with a depth as long as 100 .mu.m or more remains. In the method (b), in turn, the obtained value of Ra is about 3 angstroms and the polishing flaws decrease in depth to about 50 angstroms or less, but the waviness of the substrate remains. Also, the polishing speed is low, which requires a polishing time as long as about 5 minutes or more to attain the object Ra value. Magnetic hard disc substrates obtained after the polishing step the conventional methods inevitably have the features in that the surface roughness Ra is in a range from 7 to 15 angstroms, the surface roughness Rmax is in a range from 80 to 150 angstroms, and several scratches with a depth of 80 to 150 angstroms, some pits with a depth of 100 angstroms or less, and micro-waviness are produced. When using a suspension of known micro-sized and uniform silica particles to polish in order to avoid this situation, it takes a long time to polish because the particles are small. If erosive chemicals are added to accelerate the polishing, the silica will gel to cause the silica particles to lose their uniformity. Thus, a surface with the surface roughness Ra.ltoreq.5 angstroms and the surface roughness Rmax.ltoreq.80 angstroms cannot be achieved at present.